Small Ubiquitin-Like Modifier (SUMO) is a protein that is post-translationally conjugated to target proteins to alter their function, cellular location, and stability. In yeast there is one SUMO protein (Smt3), while in humans there are four SUMO homologs (SUMO-1, SUMO-2, SUMO-3, and SUMO-4). SUMO conjugation involves the initial activation of one of the SUMO homologs by proteolytic cleavage of C-terminal amino acid residues to reveal the mature Gly-Gly terminus, which is then adenylated, loaded onto the SUMO E1 enzyme, and transferred to the SUMO E2 enzyme. SUMO E2 (also known as Ubc9) is able to directly conjugate SUMO to target proteins alone, or in concert with SUMO E3 chaperones that facilitate SUMO E2 interaction with the target proteins. SUMOylation (conjugation of SUMO to a target protein) is a critical cellular process that impacts nuclear-cytosolic transport, transcriptional regulation, apoptosis, protein stability, response to stress, and progression through the cell cycle.